General Information

What is a threaded fitting?

This is a method of joining pipe systems together.


The sizes of pipes, fittings, valves and flanges are generally referred to by their nominal sizes (Nominal Pipe Size or NPS).

This can either be in metric units (DN - Diametre Nomimal - mm) or imperial units (inches).

A 1” threaded fitting is also known as a 25mm threaded fitting.

Note: the naming convention refers to the inside diameter (ID) of the fitting.

How to size a threaded fitting

The outside diameter for a 1” BSP threaded fitting is not 1” or 25mm but approximately 33mm. Once you have the Outside Diameter (OD) measurement for your fitting, consult the table below for its nominal size.

Nominal Pipe Size - NPS Outside Diameter
DN  (mm) Inch mm Inch
6 1/8” 9.728 mm 0.383
8 1/4” 13.157 mm 0.518
10 3/8” 16.662 mm 0.656
15 1/2” 20.995 mm 0.825
20 3/4" 26.441 mm 1.041
25 1” 33.249 mm 1.309
32 1-1/4” 41.91 mm 1.650
40 1-1/2” 47.803 mm 1.882
50 2” 59.614 mm 2.347
65 2-1/2” 75.184 mm 2.960
80 3” 87.884 mm 3.460
100 4” 113.03 mm 4.450
125 5” 138.43 mm 5.450
150 6” 163.83 mm 6.450

Different thread types

Two thread types are distinguished - BSP and NPT. The difference has more to do with the country of origin/ your location than the application. BSP threads are common across UK, Europe, Australia, New Zealand, South Africa, and most other countries. NPT are the standard pipe threads in USA and Canada.

Summarised differences for the thread types

Thread Acronym Name Series Peak & Valleys Thread Angle Also referred to as…
BSP British Standard Pipe Standard & Tapered Rounded 55° Whitworth Thread (after designer Joseph Whitworth)
NPT National Pipe Thread Standard & Tapered Flat 60° Sellers Thread (William Sellers established US Standard)
Threads can be either tapered or parallel. In the case of BSP threads:
  • Parallel (BSPP)
  • Tapered (BSPT)

Tapered vs Parallel

Tapered threads are utilised because a taper thread can be pulled tightly to create a fluid or air-tight seal. The male threads seal against the female thread.

This will not be the case for parallel or straight threads. These have been designed to hold pieces together but not to create a seal. In such cases a washer or O-ring is needed to affect a seal.

Common Questions

What threads are commonly used in Australia?

BSP thread is common in Australia and the commonwealth countries.

How to size a BSP threaded fitting?

Measure the outside diameter of your male threaded fitting and consult the table above